Once you access the parent movie clip, simply index into the child. ActionScript 2 does not require you to use the eval function for looking up dynamic properties. Simply use the object and the [] (array) operators to index the desired variable.

If your "row" objects are in the root of the current movie clip, you could simply use _root[ "row" + ActiveRow ][ "let" + i ].

However, since you already have the initial movieclip via eval("row"+ActiveRow), you can use this object to get the next level down. For example, eval("row" + ActiveRow)[ "let" + i ].

Flash borrows heavily from JavaScript, and like JavaScript, every object is essentially a hash table. Using the dot operator is equivalent to using the [] (array) operator with a fixed string.